Conventionally, an optical pickup for use in optical disk drives is structured such that a light emitted from a semiconductor laser is collected with a lens and radiated onto an optical disk, and the light reflected by the optical disk with optical power modulated by a pit indicating a signal is received by a light receiving device. An electric signal from the light receiving device is processed in a signal processing circuit, and a data signal written onto the optical disk is detected, as well as a focus signal for controlling a focus of the lens and a servo signal for controlling a light collecting position on the optical disk are detected. As the light receiving device, a so-called split type light receiving device composed of a plurality of light receiving parts is employed for detecting the data signal, the focus signal and the servo signal.
In recent years, optical disk drives using a blue semiconductor laser as a substitute for infrared and red semiconductor lasers are being developed to support higher density data that are written onto the optical disk. The split type light receiving device for use in such optical disk drive includes a conventional one shown in FIGS. 7A and 7B (Japanese unexamined patent application No. 2001-148503). FIG. 7A is a plan view showing a split type light receiving device while FIG. 7B is a cross sectional view taken along the arrow line D—D′ of FIG. 7A. The split type light receiving device is structured such that a plurality of N type diffusion layers 601, 601 as cathodes are provided on a P type semiconductor layer 600 to form light receiving parts. On the surface of the light receiving device on the side of the light receiving parts, two films composed of a silicon oxide 604 and a silicon nitride 605 are disposed to constitute an antireflection film structure 603.
The antireflection film structure 603 composed of the silicon oxide 604 and the silicon nitride 605 effectively reduces the reflectance of incident light by appropriately selecting each film thickness according to the wavelength of the incident light. Generally, combining a plurality of films different in kind as shown above makes it possible to obtain an antireflection film with a relatively small thickness and low reflectance. For example, in the case of red light with wavelength of 650 nm, the film thickness of the silicon oxide is set at 50 nm while the film thickness of the silicon nitride is set at 30 nm, so that the reflectance in the antireflection film structure 603 can reach almost 4%. Further, in the case of blue light with wavelength of 400 nm, the film thickness of the silicon oxide is set at 10 nm while the film thickness of the silicon nitride is set at 39 nm, so that the reflectance in the antireflection film structure 603 can reach almost 0%.
Further, in the vicinity of the surface of the P type semiconductor layer 600 and between a plurality of the light receiving parts, a P type diffusion layer 602 with impurity concentration of about 1E18 cm−3 to 1E19 cm−3 is disposed so as to prevent leak current between cathodes caused by positive charges that are stored in the interface between the silicon oxide 604 and the silicon nitride 605 and in the silicon nitride 605 of the antireflection film structure 603.
However, the above-stated conventional light receiving device has a problem that leak current between cathodes caused by positive charges stored on the surface of the silicon nitride 605 cannot be prevented. More specifically, during a reliability test and the like after the light receiving device is produced, if a power supply voltage is applied to the cathode of the light receiving device for a long time, then electric charges present in the silicon nitride 605 of the antireflection film structure 603 are redistributed by Pool-Frenkel current. Moreover, electric charges influenced by static charges and contamination are stored on the surface of the silicon nitride 605. Leak current flows between cathodes by these electric charges. FIG. 8 is a graph showing changes in leak current between cathodes as a reverse bias voltage of the light receiving device is changed, where the horizontal axis represents a reverse bias voltage (V) as a power supply voltage applied to the light receiving device, and the vertical axis represents current (A) between cathodes. Further as shown in FIG. 9, corresponding to the length of a period of time during which the supply voltage is applied, the leak current between cathodes is increased. In FIG. 9, the horizontal axis represents elapsed time (hour) after the reverse bias voltage is applied, while the vertical axis represents leak current (A) between cathodes.
The reason why the leak current flows between cathodes will be described with reference to schematic views shown in FIG. 10 and FIG. 11. FIG. 10 is a schematic cross sectional view showing the light receiving device of FIG. 7B after a long-time reliability test is carried out. As shown in FIG. 10, positive charges 610 are stored on the surface of the antireflection film structure 603, and the stored positive charges 610 generate inversion charges 611 in the vicinity of the surface of the P type semiconductor layer 600 and between the N type diffusion layers 601, 601. FIGS. 11A and 11B are views showing redistribution of electric charges caused by Pool-Frenkel current in the light receiving device of FIG. 7B. First, as shown in FIG. 11A, during a production process of the light receiving device, the silicon nitride 605 is damaged by plasma or the light receiving device is formed into a chip for a wire bonding step performed after production of the light receiving device, by which positive charges 612 and negative charges 613 are generated in the silicon nitride 605. Then, when a voltage is applied to the N type diffusion layers 601, 601 during a reliability test, the positive charges 612 in the silicon nitride 605 are accumulated in the across-the-width center of the silicon nitride 605 as shown in FIG. 11B, and these positive charges 612 generate inversion charges 614 in a portion of the P type semiconductor layer 600 between the N type diffusion layers 601, 601. Here, the voltage applied to the N type diffusion layers 601, 601, i.e., a reverse bias voltage of the cathode, generates a repulsive force in the silicon nitride 605, as a result of which a number of positive charges 612 are concentrated in a region of the silicon nitride 605 corresponding to a portion between the cathodes.
As shown in FIG. 10 and FIG. 11, the generated inversion charges 611, 614 are also generated in the P type diffusion layer 602 positioned between the N type diffusion layers 601, 601. These inversion charges 611, 614 cause leak current flowing between the N type diffusion layers 601, 601.
For preventing the current from flowing between the cathodes, it is necessary to decrease an inversion voltage generated by the positive charges, which is considered to be fulfilled by either increasing the impurity concentration of the P type diffusion layer 602, or increasing the thickness of the antireflection film structure 603. However, if the impurity concentration of the P type diffusion layer 602 is increased, then carriers generated upon light reception tend to recombine, resulting in degraded sensitivity of the light receiving device. If the thickness of the silicon nitride 605 is increased for increasing the thickness of the antireflection film structure 603, stress is generated in this silicon nitride 605, and the stress heightens an interface state between the P type semiconductor layer 600 and the silicon oxide 604, resulting in degradation of light receiving sensitivity. Moreover, if the thickness of the silicon oxide 604 is increased, the interface state between the P type semiconductor layer 600 and the silicon oxide 604 is heightened, causing degradation of the sensitivity of the light receiving device. Therefore, the film thickness of the silicon oxide 604 should be about 300 nm or less while the film thickness of the silicon nitride 605 should be about 50 nm or less. However, these film thicknesses cannot prevent leak current after application of the supply voltage.
Accordingly, an aspect of the present invention is a light receiving device which causes almost no leak current even after continual operation for a long time and which is free from degradation of sensitivity.